Check out the answer to a similar question which should help:
http://serverfault.com/questions/1014/can-anyone-recommend-a-website-monitoring-service
To answer your question more specifically however, every major monitoring service will provide performance metrics in addition to basic uptime/availability notifications. For example, the Top Tier (Webmetrics, Keynote, Gomez) will give you waterfall graphs that show you a timeline showing how every item on your page loads from all around the world.
The most important thing to look for when you look for a vendor in this area is the monitoring technology. You want to make sure that the monitoring happens with a real browser, not an emulated browser. You want to see the performance based on how real users will be seeing your site, including the maximum number of simultaneous threads, how javascript and css are handled, and the general performance that a real browser gives you. A few of the vendors have been known to claim they are monitoring with a real browser, when in reality they aren't, or it's a lot more expensive.
To determine which is the best, I would recommend you get a trial with each one and see for yourself.
Note that this kind of information, especially if you are looking for performance from locations around the world, is what separates the top tier from the bottom.